Many naturally occurring and synthetic biomolecules are immunogens that elicit an immune response without the need for conjugation. However, to elicit an immune response, a molecule must contain an antigenic determinant or epitope. This epitope must be of sufficient size to initiate lymphocyte activation necessary to make antibodies. In general small molecules are not good immunogens. On the other hand, when small molecules, such as sugars, oligonucleotides, or peptides, are conjugated or attached to macromolecules, also called carrier molecules, they can become immunogenic.
In immunology, small molecules or compounds are also called haptens. Haptens and their corresponding conjugates are heavily used for the development and production of sensitive, qualitative and quantitative assays, such as enzyme-linked immunosorbent assays (ELISAs). The density of the hapten on the carrier molecule is important for the successful elicitation of a good or strong immune response needed for the development of immunoassays.
Lemus R, Karol MH.; Conjugation of haptens. Methods Mol Med. 2008;138:167-82. doi: 10.1007/978-1-59745-366-0-14.